Methods for fabricating a substrate

ABSTRACT

A method is provided for fabricating a substrate for optics, electronics, or optoelectronics. This method includes the steps of transferring a seed layer to a support layer, depositing a working layer onto the seed layer to form a composite substrate and detaching the working layer and the seed layer from the support to form a substrate. Advantageously, the support substrate comprises a material having a thermal expansion value of about 0.7 to 3 times the coefficient value of the working layer.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of International Application PCT/FR01/03714 filed Nov. 26, 2001, the entire content of which is expressly incorporated herein by reference thereto.

FIELD OF THE INVENTION

[0002] The invention relates to methods for fabricating substrates, in particular for optics, electronics, or optoelectronics, and also to substrates obtained by such methods. More particularly, the substrates are also suitable for use in fabricating light-emitting and laser diodes.

BACKGROUND OF THE INVENTION

[0003] In the technology field of optics, electronics, and optoelectronics, it is often desirable to obtain substrates that comprise a working layer. Two types of methods are currently known for making such substrates. One method includes transferring a working layer from a source substrate onto a support substrate. The other method includes depositing a working layer onto a support substrate by deposition techniques such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOVCD), and the like.

[0004] Source substrate materials from which a working layer can be transferred, however, are very difficult to obtain or are otherwise not available. This applies in particular to monocrystalline gallium nitride, which is not available in the form of a solid single crystal of quality, nor is it available with a diameter that is satisfactory, and not available at a reasonable price. Consequently, monocrystalline gallium nitride is typically grown only by a heteroepitaxial technique.

[0005] Furthermore, growth of a working layer on a substrate by deposition techniques is not yet satisfactory. There are known techniques for growing a working layer on a seed layer itself carried by a support, in which there is often a need to remove the support substrate in order to obtain the final product. Although techniques for removing the support are known, there are drawbacks. For example, FR 2,787,919A, which is incorporated herein by reference thereto, describes eliminating such a substrate by a mechano-chemical thinning technique. However, all techniques of removing the support by etching or some equivalent technique are undesirable since they lead to significant losses of material, which often times is expensive. U.S. Pat. No. 6,114,188, incorporated herein by reference, also describes a technique for separating a complex transition metal oxide (CTMO) film made by deposition, in which a special treatment is performed on the native substrate from which the film is to be grown, and then the deposited film is detached from the native substrate. Nevertheless, that technique is also undesirable since it runs the risk of compromising proper initiation of film growth and thus leads to either a loss of yield or to a deposited layer of poor quality.

[0006] Additionally, it is known that sapphire, silicon carbide and {111} silicon can be used as seed substrates for the deposition of gallium nitride by heteroepitaxy. However, sapphire is an electrical insulator, which is a disadvantage in certain applications, and monocrystalline silicon carbide presents the drawbacks of being expensive and difficult to obtain in large diameters. Furthermore, as sapphire is an electrical insulator, if it is retained in the form of a solid support, then it becomes necessary for any electrodes needed for the intended application of the working layer to be provided solely on the working layer itself, which can give rise to problems of available space (for example, if two electrical contacts are to be made on a front face, i.e. on the free surface of the working layer).

[0007] In prior art techniques, monocrystalline silicon carbide or sapphire were used to serve both as a support and as a growth seed for the working layer. One drawback from such prior art techniques exists when the working layer is used to form light-emitting diodes (LEDs), using a solid silicon carbide or sapphire support means, namely, that it is not possible to satisfactorily control the positions of electrical contacts, the extraction of the light emitted by the diode, or the use of a reflecting surface, etc

[0008] Although {111} silicon may be ideal for use as a substrate due to its very widespread use, the fact that it is inexpensive and available in large diameters, problems have arisen during attempts to deposit gallium nitride on {111} silicon using the standard technique of MOCVD at about 1000° C. to 1100° C. Such problems include dislocations forming in he thin layer of gallium nitride at a concentration in excess of 108 per square centimeter (cm²). Moreover, if {111} silicon is used as a support, i.e. in thick form, then cracking is observed in the working layer because of poor matching in terms of thermal expansion.

[0009] In other prior art techniques, attempts have been made to deposit gallium nitride directly on solid gallium nitride or indeed on neodymium gallate or on indium gallate. However, solid gallium nitride is expensive and those techniques are not mature.

[0010] Thus, improvements in such methods for fabricating a substrate are desired. Also needed is a method that overcomes the difficulties associated with substrate materials. Certain new and useful improved methods are provided by the present invention.

SUMMARY OF THE INVENTION

[0011] The invention provides new ways for fabricating substrates for use in the fields of optics, electronics, or optoelectronics. In particular, in accordance with the invention a method is provided for fabricating a substrate generally comprising transferring a seed layer onto a support, depositing a working layer on the seed layer to form a composite substrate, and detaching the seed layer and the working layer from the composite substrate to form a substrate. Advantageously, the support substrate comprises material having a thermal expansion value of about 0.7 to 3 times the coefficient value of the working layer. Preferably, the seed layer comprises material suitable for accommodating the thermal expansion of the support layer and the working layer.

[0012] Also in accordance with the invention, a seed layer is transferred onto a support by molecular adhesion in an adhesion interface, a working layer is deposited onto the seed layer to form a composite substrate, and the seed layer and the working layer are detached from the composite substrate by the application of stress. Advantageously, detachment of the support from the seed layer and working layer may be accomplished by the application of stress which therefore provides for the possibility of recycling and reusing the support substrate.

[0013] The method may also include a source substrate comprising a seed layer and a weakened zone. The seed layer may be detached from the source substrate at the weakened zone, and transferred onto the support substrate.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0014] Other aspects, objects, and advantages of the invention will appear on reading the following detailed description, and the invention will also be better understood with the help of the accompanying drawings.

[0015]FIG. 1 is a diagram showing the steps in an implementation of the method in accordance with the invention;

[0016]FIG. 2 is a diagram of the steps in another implementation of the method in accordance with the invention;

[0017]FIG. 3 is a diagram of the steps in yet another implementation of the method of the invention; and

[0018]FIG. 4 is a diagrammatic perspective view of an intermediate support with four seed layers, of the kind that can be used in a variant of the method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In accordance with the invention, the method comprises transferring a seed layer onto a support substrate, depositing a working layer on the seed layer to form a composite substrate; and detaching the seed layer and the working layer from the composite substrate.

[0020] The seed layer in accordance with the invention may comprise material such as sapphire, silicon carbide, zinc oxide, silicon, gallium nitride, neodymium gallate, lithium gallate, or any combination thereof. Also included are other materials commonly known in the art.

[0021] Preferably the seed layer includes a crystal lattice parameter that is sufficient for growing a working layer onto the seed layer that has, wherein the working layer has a dislocation concentration less than about 10⁷/cm².

[0022] Also in accordance with the invention, the method comprises a source substrate including a seed layer and a weakened zone. In this embodiment, the seed layer is detached from the source substrate at the weakened zone, and transferred onto a support substrate. The detached seed layer is transferred onto the support substrate, for example, by molecular adhesion at an adhesion interface. A working layer is deposited onto the transferred seed layer to form a composite substrate. The seed layer and the working layer are detached from the composite substrate at the adhesion interface.

[0023] The depositing step may include depositing the seed layer onto the support substrate by epitaxy. However, other methods as known in the art may be used. The seed layer and the working layer are detached from the composite substrate by the application of stress, for example.

[0024] Optionally, the invention may further include applying a first bonding layer onto the source substrate, and applying a second bonding layer onto the support substrate. Preferably, the source substrate includes implanted atomic species to a predetermined depth to form a weakened zone. The first and second bonding layers may be contacted to attach the source and support substrates, and then the seed layer can be detached from the source layer at the weakened zone and then transferred onto the support substrate. Thereafter, the working layer may be deposited onto the seed layer, which has been transferred to the support substrate.

[0025] Advantageously, the detaching step, as mentioned above, comprises application of stress, such as mechanical stresses, thermal stresses, electrostatic stresses, laser irradiation stresses, or a combination thereof. Thus, if desired the support may be recycled and reused.

[0026] The support substrate preferably comprises a material having a coefficient of thermal expansion of about 0.7 to 3 times the coefficient of the working layer, and the seed layer is suitable for accommodating the thermal expansion of the support and of the working layer. Preferably, the values of the thermal expansion coefficients herein relate to those in a plane parallel to the plane of the working layer. Advantageously, the material selected to form the support presents thermal expansion coefficients that reduce or even eliminate significant tension or compression stresses of the kind that arise during the variations in temperature that are inherent to growing the working layer, or that occur on returning the substrate formed in this way to ambient temperature.

[0027] Importantly, tolerance to the differences of the thermal expansion coefficients of the working layer and the support substrate materials, increases when the difference leads to compression in the working layer, as opposed to when it leads to stretching of the layer. Thus, in compression, the thermal expansion coefficient of the support material can be several times greater than that of the working layer. However, in extension, the thermal expansion coefficient of the support material is preferably not less than 0.7 times the coefficient of the working layer.

[0028] Preferably, the seed layer is suitable for adapting to the thermal expansions imposed by the support and/or the working layer. For this purpose, the seed layer has a thickness that is small enough to enable it to be deformed so as to accommodate the dimensional variations due to thermal expansion of the support and/or of the working layer. This thickness depends on the material constituting the seed layer and on the respective materials of the support and of the working layer. Typically, for a silicon carbide support having thickness of 300 microns (μm) and a gallium nitride working layer having thickness of several microns, a monocrystalline silicon carbide seed layer should have thickness that is less than 0.5 μm, and that is preferably less than 1000 angstroms (Å).

[0029] Advantageously, as mentioned above, the material comprising the seed layer also presents lattice parameters such that the working layer can be grown epitaxially on the seed layer with dislocations in the working layer at a concentration of less than 10⁷/cm². The person skilled in the art knows how to perform such epitaxial growth by selecting parameters and orientations both for the seed layer and for the working layer.

[0030] In accordance with the invention, the method provides, as shown in FIG. 1, a final substrate 14 comprising a working layer 16 on a seed layer 2. In one embodiment, the method comprises applying a bonding layer 10 on a surface of a source substrate 6, which is subjected to implantation of atomic species. A second bonding layer 11 is preferably applied to a surface of a support 12. The implanted atomic species are implanted to a predetermined depth in the source substrate 6 so that a zone of weakness 8 is formed. The bonding layers are preferably contacted 100, and the seed layer is detached from the source substrate 6 via the zone of weakness 8, and 300 and a working layer is deposited 16 on the surface of the seed layer 2 corresponding to the zone of weakness 8. The steps of forming the bonding layer 10 and of implanting the atomic species can be performed in the order specified above or in another order.

[0031] As used herein, the term “atomic implantation” is used to cover any bombardment using atomic or ionic species and suitable for introducing these species into a material so as to achieve a concentration maximum of these species in the material, said maximum being situated at a determined depth relative to the bombarded surface. The atomic or ionic species are introduced into the material with energy distributed around a maximum. Atomic species can be implanted in the material using an ion beam implanter, a plasma immersion implanter, etc. The term “atomic or ionic species” is used to mean an atom in ionic, neutral, or molecular form, or molecules in ionic or neutral form, or indeed a combination of different atoms or molecules in ionic or neutral form.

[0032] Alternatively, an intermediate layer such as an insulating layer may be applied to form a substrate comprising a semiconductor on insulation. By way of example the intermediate layer can be made of diamond, of fine oxide (500 Å thick), and the like, and may be applied between the working layer 16 and/or the seed layer 2 (if it is conserved) and the support 12, or the thick layer 4 as described below.

[0033] The implanting atomic species step and the detaching the seed layer step 200 are described in French patent No. FR 2,681,472, which is incorporated by express reference thereto. Although other commonly known methods may also be used. The steps of forming the bonding layers 10 and 11 correspond, by way of example, to forming a layer of amorphous material using any of the methods known to the person skilled in the art.

[0034] An optional step of preparing the surface of the seed layer 2 that is to receive the working layer 16 may be performed. Such preparation of the surface include but are not limited to polishing operations, annealing operations, smoothing annealing operations (e.g. under hydrogen), annealing operations for reinforcing the adhesive interface between the bonding layers 10 and 11, sacrificial oxidation operations (oxidation followed by elimination of the oxidized material), etching operations, and the like.

[0035] Table 1 below summarizes the lattice parameters and the thermal expansion coefficients of various materials that are suitable for use in implementing the method of the invention, either as a material for the working layer or as a material for the seed layer or as a material for the support substrate. However, these examples are illustrative and should not be construed as limiting the features of the invention. TABLE 1 GaN AlN Al₂O₃ Si 6H—SiC (W) (W) (H) (C) (W) Lattice a = 3.189 a = 3.112 a = 4.758 c = 5.430 a = 3.08  parameters c = 5.185 c = 4.982 c = 12.99 c = 15.12 (Å) Thermal 5.59 4.15 7.5 2.6 4.20 expansion 3.17 5.27 8.5 4.68 coefficients along a or c (×10⁻⁶K⁻¹) Thermal 1.3 2.5 0.5 1.5 4.9 conductivity (W/cm · K) Maximum 4 8 2 diameter currently available (inches) Quality of Excellent Excellent Variable substrates available Relative 10 1 125 prices for 2-inch substrates (arbitrary units)

[0036] Preferably, the working layer comprises material such as gallium nitride, aluminum nitride, and gallium and aluminum nitride (and in general manner, semi conductive nitrides having a large band gap).

[0037] The seed layer preferably comprises materials such as sapphire, silicon carbide, zinc oxide, and {111} silicon. The seed layer may also be selected to obtain a precise crystal structure and orientation, for example to obtain hexagonal or cubic gallium nitride, or if an Si face or a C face is selected to deposit the working layer on a seed layer of silicon carbide; the seed layer is made of gallium nitride of very high quality, i.e. having fewer than 10⁶ dislocations per square centimeter, e.g. gallium nitride obtained by the epitaxial lateral overgrowth (ELOG) technique.

[0038] The support substrate material preferably comprises amorphous materials, polycrystalline materials, and sintered materials. More preferably the support comprises materials such as polycrystalline silicon carbide, monocrystalline silicon carbide, polycrystalline aluminum nitride, polycrystalline gallium nitride, and monocrystalline gallium nitride having a high concentration of dislocations (greater than 10⁷/cm²). The materials for both the seed layer and the support may be have the same chemical composition.

[0039] As mentioned above, the seed layer may be dissociated from a source substrate at a weakened zone. In this respect, the seed layer taken from the source substrate may be assembled to the support and then eroded via its free face so as to obtain a seed layer of desired thickness (for this purpose it is possible to use a technique taken from the technique used for obtaining substrates and of the type known in the art as bond and etch back silicon on insulator (BESOI)). The dissociation of the seed layer may be accomplished at least in part by heat treatment, applying mechanical stresses, chemical etching, or a combination of at least two of these operations. Alternatively, other known methods in the art may be used.

[0040] Table 2 below summarizes examples of materials that are suitable for use in implementing the methods of the invention described above. These are illustrative and should not be construed as limiting the features of the invention. TABLE 2 Seed layer 2 Bonding (typically layers 10, Support 12 1000 Å 11 (typically (typically 300 μm Working layer 16 thick) 1 μm thick) thick) GaN or AIN or Mono SiC SiO₂ or Si₃N₄ Poly SiC or mono AlGaN or GaInN or SiC or sapphire or SiC or other poly AIN or poly GaN GaN or AIN or {111} Si SiO₂ or Si₃N₄ Poly SiC or mono AlGaN or GaInN or SiC or sapphire or SiC or other poly AIN or poly GaN GaN or AIN or Sapphire SiO₂ or Si₃N₄ Poly SiC or mono AlGaN or GaInN or SiC or sapphire or SiC or other poly AIN or poly GaN GaN or AIN or Mono GaN SiO₂ or Si₃N₄ Poly SiC or mono AlGaN or GaInN or SiC or sapphire or SiC or other poly AIN or poly GaN GaN or AIN or NdGaO₂ or SiO₂ or Si₃N₄ Poly SiC or mono AlGaN or GaInN or LiGaO₃ SiC or sapphire or SiC or other poly AIN or poly GaN

[0041] In the above table, as in the tables below, the term “mono” is used to mean “monocrystalline” and the term “poly” is used to mean “polycrystalline”.

EXAMPLE 1

[0042] In the example corresponding to the first row of Table 2, a working layer 16 is made of gallium nitride on a seed layer 2 constituted by monocrystalline silicon carbide, itself on a polycrystalline silicon carbide support 12, with bonding layers 10 and 11 of silicon carbide being interposed between the support 12 and the seed layer 2. By way of example, the seed layer 2 is 1000 Å thick. The support 12 is 300 μm thick, for example.

[0043] The structure comprising a stack of the seed layer 2 of monocrystalline silicon carbide, the two bonding layers 10 and 11 of silicon oxide, and the support 12 of polycrystalline silicon carbide is made by a layer transfer method known to the person skilled in the art (e.g. see an application of the Smart-Cut® method in French patent No FR 2,681,472).

[0044] The working layer 16 can be made by chemical vapor deposition (CVD), by high temperature chemical vapor deposition (HTCVD), by MOCVD, by MBE, or by hydride vapor phase epitaxy (HVPE), see for example the documents “GaN bulk substrates for GaN-based LEDs and LDs”, by O. Oda et al., Phys. Stat. Sol. (a), No. 180, p.51 (2000), or other equivalent techniques.

[0045] The use of silicon oxide for the bonding layer 10 makes it easier to take the seed layer 2 from the source substrate 6. The planarized deposit of silicon oxide makes it possible to eliminate surface irregularities and to perform a polishing step, a planarizing step, a cleaning step, a chemical preparation step, and a step of bonding said silicon oxide onto the silicon oxide of the bonding layer 11 formed on the support 12 using conventional techniques that are easy to implement. By way of example, the set of bonding layers 10 and 11 is one micron thick.

EXAMPLE 2

[0046] In this example (second row of above table), a structure is provided that is equivalent to that of Example 1, except that the seed layer 2 of silicon carbide is replaced by a seed layer 2 of {111} silicon.

[0047] The thickness of the {111} silicon is preferably limited to less than 3000 Å so as to enable it to adapt without cracking to the thermal expansion that will take place during the various operations mentioned above.

EXAMPLE 3

[0048] In this example (third row of the above table), a structure is made that is equivalent to that of Examples 1 and 2, except that the seed layer 2 is made of sapphire.

[0049] Sapphire is another material that is known for permitting good epitaxy of gallium nitride.

EXAMPLE 4

[0050] In this example (fourth row of the above table) a structure is made that is equivalent to that of Examples 1 to 3, except that the seed layer 2 is made of monocrystalline gallium nitride.

EXAMPLE 5

[0051] In this example (fifth row of the above table), a structure is made that is equivalent to that of Examples 1 to 4, except that the seed layer 2 is made of neodymium gallate or of lithium gallate.

[0052] Numerous variants of the above examples are encompassed by and are in accordance with the present invention. For example, the silicon oxide in one of the bonding layers 10 and 11, or the silicon oxide in both bonding layers 10 and 11 can be substituted by some other material, for example silicon nitride (Si₃N₄). This material can withstand higher temperatures than silicon oxide. This advantage is particularly advantageous in the context of optimizing deposition of the working layer 16 in order to form a monocrystalline layer of good quality or indeed when it is desired to increase the rate of deposition. Silicon nitride also has the advantage of limiting or even avoiding diffusion of gallium into the support 12.

[0053] In yet another variant of the method of the invention, the working layer 16 of gallium nitride is substituted by a working layer 16 of aluminum nitride, of silicon carbide, an alloy of aluminum and of gallium, and alloy of gallium and indium, or some other compound. The working layer 16 of gallium nitride can also be substituted by a multilayer structure constituting a stack of layers of the gallium nitride, aluminum nitride, gallium and indium nitride, etc. types, possibly having different kinds of doping, etc.

[0054] In yet other variants, the polycrystalline silicon carbide support 12 is substituted by monocrystalline silicon carbide (in particular when the support 12 can be recycled as mentioned below), sapphire, polycrystalline aluminum nitride, or polycrystalline gallium nitride.

[0055] After growing the working layer 16, the support 12 is removed, possibly after reinforcing the structure as a whole, where necessary for questions of strength, by means of some other support either by direct adhesion, or by forming said other support by depositing it on the working layer, etc.

[0056] The support 12 must then be capable not only of withstanding the conditions under which the working layer 16 is grown, but is advantaged by being suitable for removal. The technique selected for withdrawing the intermediate support 12 can determine the material selected for constituting it. If it is to be sacrificed by etching or by mechanical or chemical removal, then the etching and removal steps and also the intermediate support 12 itself must be as low cost as possible. Under such circumstances, the support 12 should be made of polycrystalline aluminum nitride.

[0057] In accordance with the invention, stresses such as mechanical, thermal, electrostatic, laser irradiation, etc. are used to cause the composite substrate to split into two portions situated on either side of the adhesion interface. Under such circumstances, the support 12 can be made of monocrystalline silicon carbide since it is not consumed and can be reused.

[0058] In another variant, all or some of the components on the working layer 16 are made either before or after removing the support 12.

[0059] In another embodiment of the invention, the method of the invention, and as shown in FIG. 2, provides a structure comprising a working layer 16 on a seed layer 2 itself on a support 12 with bonding layers 10 and 11 interposed between the seed layer 2 and the support 12. A thick layer 4 is deposited on the free surface of the working layer 16 and the support 12 is removed, possibly together with the seed layer 2. The thick layer 4 then serves in particular to form a support for the working layer 16, after the support 12 has been removed.

[0060] Table 3 below summarizes examples of materials that can be used in the context of this second implementation of the method of the invention. These examples are purely illustrative and should not be construed as limiting. TABLE 3 Bonding Working layer layers 16 Seed layer 2 10, 11 Support 12 Thick layer 4 GaN or AIN or {111} Si or SiO₂ or Poly SiC or Diamond or AlGaN or mono SiC or Si₃N₄ poly AIN or poly SiC or GaInN or SiC GaN or sapphire or GaN or AIN or or other sapphire or mono SiC or boron nitride or NdGaO₂ or poly GaN metal (copper) LiGa₃

EXAMPLE 6

[0061] A seed layer 2 of monocrystalline {111} silicon is made on a polycrystalline silicon carbide support 12 with silicon oxide bonding layers 10 and 11 between them. Thereafter a working layer 16 of monocrystalline gallium nitride is deposited by MOCVD and a thick layer 4 of diamond is deposited on the free face of the monocrystalline gallium nitride of the working layer 16.

[0062] Thereafter the resulting structure is subjected to treatment suitable for detaching the composite substrate constituted by the working layer 16 from the composite substrate constituted by the support 12 and the seed layer 2. This treatment comprises using mechanical, thermal, electrostatic, etc. stresses to cause two portions to become separated on either side of the adhesion interface.

[0063] This example presents the advantage of making it possible to use a support 12 whose surface that is to receive the seed layer 2 is poorly finished, but once the working layer 16 of GaN has been formed by virtue of the seed layer 2 of {111} silicon, it is possible to make a final support (the thick layer 4) for the working layer 16 that has properties adapted to using said working layer 16 (in this case, with diamond, it is the good thermal conductivity and electrical insulating properties that are desired, e.g. for microwave applications), and it is also possible to ensure a high quality interface between the thick layer 4 and the working layer 16, e.g. to obtain better heat conduction.

[0064] This embodiment may have variants such as the seed layer 2 of {111} silicon can be replaced by monocrystalline silicon carbide, sapphire, neodymium gallate, or lithium gallate; the bonding layers 10 and 11 of silicon oxide can be replaced by silicon nitride; the polycrystalline silicon carbide support 12 can be replaced by monocrystalline silicon carbide or by sapphire; and the thick layer 4 of diamond can be replaced by polycrystalline silicon carbide, by polycrystalline gallium nitride (e.g. deposited by HVPE), by boron nitride, or by a metal such as copper, etc. (e.g. deposited as a thick layer by electrolysis).

[0065] The thickness properties of the thick layer 4 can be of importance, for example when it is desired to make electrical contact with the rear face of the final substrate 14 or when it is essential to be able to evacuate the heat generated by components made on the working layer 16, or indeed when it is desired to improve extraction and control of light emitted by a diode or a laser made in the working layer 16. It will then be understood that the properties selected for the thick layer 4 provide a degree of freedom in substrate fabrication methods that is particularly advantageous when making substrates for optics, electronics, optoelectronics, etc. It is also possible to add a degree of freedom in substrate fabrication methods by providing preparation steps (known in themselves to the person skilled in the art) in order to enable the thick layer 4 to be subsequently separated from the working layer 16.

[0066] Similarly, these variants can be transposed to situations in which a working layer 16 is made of aluminum nitride, of silicon carbide, an alloy of aluminum and gallium, or of other compounds, instead of making the working layer 16 out of gallium nitride as described above. The gallium nitride working layer 16 can also be a multilayer structure stacking layers of the gallium nitride, aluminum nitride, etc. type, possibly with doping of different kinds, etc.

[0067] In yet another embodiment of the invention, as shown in FIG. 3, a structure is made in which the thick layer 4 is deposited after the working layer 16 and the seed layer 2 have been separated, contrary to that which is described above with reference to the second implementation of the invention. The thick layer 4 can then either be deposited on the same side as the free face of the working layer 16 or else it can be deposited on the same side as the seed layer 2, either on said seed layer 2 or on the corresponding face of the working layer 16, if the seed layer 2 is removed either together with the support 12 or after the support 12 has been removed.

[0068] Another embodiment of the method of the invention is described below with reference to three examples. The materials used in the context of these three examples are summarized in Table 4, and they correspond to the materials of Table 3. TABLE 4 Bonding Working layer layers 16 Seed layer 2 10, 11 Support 12 Thick layer 4 GaN or AIN or Mono Sic or SiO₂ or Mono SiC or AIN or GaN or AlGaN or {111} Si or Si₃N₄ poly Sic or poly SiC or GaInN or SiC sapphire or poly AIN or diamond or GaN or sapphire or boron nitride or NdGaO₂ or poly GaN metal (copper) LiGa₃ GaN or AIN or Mono Sic or SiO₂ or Mono SiC or AIN or GaN or AlGaN or {111} Si or Si₃N₄ poly Sic or poly SiC or GaInN or SiC sapphire + poly AIN or diamond or etching GaN sapphire or boron nitride or or NdGaO₂ poly GaN metal or LiGa₃ +etching a Mono Sic or SiO₂ or Mono SiC or Poly SiC or portion of the {111} Si or Si₃N₄ poly Sic or diamond or GaN or the sapphire + poly AIN or boron nitride or AIN or the etching GaN sapphire or metal AIGaN or the or NdGaO₂ poly GaN GaInN or the or LiGa₃ SiC

EXAMPLE 7

[0069] In this example (first row of Table 4), a structure is made comprising a monocrystalline silicon carbide seed layer 2 on a monocrystalline silicon carbide support 12 with silicon oxide bonding layers 10 and 11 between them, in the same manner as that described above for Example 1. Thereafter, a working layer 16 of monocrystalline gallium nitride is made on the free surface of the silicon carbide seed layer 2 by MOCVD. The structure obtained in this way is then subjected to treatment suitable for separating the structure constituted by the seed layer 2 and the working layer 16 from the support 12. This provides firstly a structure constituted by a gallium nitride working layer 16 covered in a seed layer 2 of monocrystalline silicon carbide, and a support 12 that is ready for recycling. A thick layer 4 of polycrystalline silicon carbide is then deposited by CVD on the seed layer 2.

[0070] The monocrystalline silicon carbide support 12 is relatively expensive, but in the present example it is recycled when implementing the method of the invention on a subsequent occasion.

EXAMPLE 8

[0071] In another example (second row of Table 4) of this third implementation of the method of the invention, the same structure is made as in Example 7, but the seed layer 2 of monocrystalline silicon carbide is withdrawn, e.g. by plasma etching, prior to forming the thick layer 4 of polycrystalline silicon carbide.

EXAMPLE 9

[0072] In yet another example of the method of the invention (third row of Table 4), a structure is made as in Example 8, except that not only is the seed layer 2 of monocrystalline silicon carbide removed, but so also is a portion of the working layer 16 of gallium nitride so as to conserve a working layer 16 that presents as few defects as possible.

[0073] The seed layer 2 of monocrystalline silicon carbide or the working layer 16 of monocrystalline gallium nitride can be subjected to various additional technological steps prior to being subjected to deposition of the thick layer 4, these steps seeking to provide some or all of the electronic components, or comprising making uniform deposits of additional films, either epitaxially or otherwise.

[0074] The polarity of the seed layer 2 of monocrystalline silicon carbide (Si face or C face) and the polarity of the working layer 16 of gallium nitride can be determined by the polarity selected for the initial source substrate 6. The method of the invention optionally includes at least one double transfer enabling polarity to be changed twice over.

[0075] Similarly, these examples can be transposed to situations in which, in accordance with the invention, a working layer 16 is made of aluminum nitride, of silicon carbide, of aluminum and gallium alloy, of indium and gallium alloy, or of some other compound, instead of making the working layer 16 out of gallium nitride as described above. The gallium nitride working layer 16 can also be a multilayer structure comprising a stack of layers of the gallium nitride, aluminum nitride, etc. types, optionally with different kinds of doping, etc.

[0076] The seed layer 2 can be made of {111} silicon or of sapphire or of neodymium gallate, or of indium gallate, etc. instead of being made of monocrystalline silicon carbide.

[0077] The support 12 can be made of polycrystalline silicon carbide or of polycrystalline silicon nitride or of polycrystalline aluminum nitride or of sapphire or of polycrystalline gallium nitride, instead of being made of monocrystalline silicon carbide. The thick layer 4 can be made of polycrystalline aluminum nitride, of diamond, or of boron nitride, instead of being made of polycrystalline silicon carbide.

[0078] In yet another embodiment of the method of the invention, a structure is made as in the above examples, except that no intermediate layers or bonding layers 10 and 11 are made. For example, a seed layer 2 is taken from a source substrate 6 of {111} silicon, and is assembled with a polycrystalline silicon carbide support 12 by direct adhesion (e.g. as described in the preceding implementations). Thereafter, one of the techniques mentioned above is used to deposit a working layer 16 of gallium nitride on the seed layer 2.

[0079] Numerous other variants to the above-described embodiments can also be envisaged without going beyond the ambit of the invention. For example, the operations described in different examples of the implementations of the method of the invention can be combined with one another.

[0080] As shown in FIG. 4, one variant consists in performing batch treatment on the seed layers 2 prior to depositing the working layer 16. Under such circumstances, the seed layers 2 are fixed on a common support 12 of large size.

[0081] Additionally, the shape of this common support 12 can be arbitrary (circular, rectangular, etc.).

[0082] Under these circumstances, the seed layers 2 can be identical or they can be different. Each of the seed layers 2 can be subjected to a separate operation of detaching the seed layer from the support 12. By way of example, the common support 12 can be a plate of polycrystalline silicon carbide covered in an oxide of silicon.

[0083] Advantageously, a stiffening substrate may be bonded to the working layer 16 of each of the various assemblies prior to the operation to detach the working layer assembly 16 on the seed layers 2 from the support 12.

[0084] Each common support 12 is recycled.

[0085] Also in accordance with the invention, the parameters governing deposition of the thick layer 4 are optimized so as to make a thick layer 4 that is monocrystalline. Even if the monocrystalline thick layer 4 is not of best quality, its quality can nevertheless be sufficient in numerous applications, whenever very high crystal quality is required only for the working layer 16.

[0086] The method of the invention is particularly advantageous when single crystals cannot be grown (as is the case of gallium nitride) or when they are difficult to grow (as is the case for silicon carbide).

[0087] The aforementioned description of the invention may be transposed to growing a working layer out of other semiconductor materials, such as indium phosphide, gallium arsenide, germanium, silicon-germanium, etc., or indeed other materials such as lithium niobate. 

What is claimed is:
 1. A method for fabricating a substrate for optics, electronics, or opto-electronics, which method comprises: transferring a seed layer on to a support substrate; depositing a working layer on the seed layer to form a composite substrate; and detaching the seed layer and the working layer from the composite substrate to form a substrate; wherein the support substrate comprises a material having a thermal expansion value of about 0.7 to 3 times the coefficient value of the working layer.
 2. The method of claim 1, wherein the seed layer is transferred onto the support layer by molecular adhesion in an adhesion interface.
 3. The method of claim 1, wherein the working layer is deposited on the seed layer by chemical vapor deposition, high temperature chemical vapor deposition, hydride vapor phase epitaxy, epitaxy, metal organic chemical vapor deposition, or molecular beam epitaxy.
 4. The method of claim 1, which further comprises detaching the seed layer and the working layer from the composite substrate by the application of stress at the adhesion interface, wherein the stress is selected from the group consisting of mechanical stress, thermal stress, electrostatic stress and laser irradiation stress, or any combination thereof.
 5. The method of claim 1, wherein the working layer is gallium nitride.
 6. The method of claim 1, wherein the seed layer comprises a material from the group consisting of sapphire, silicon carbide, zinc oxide, silicon, gallium nitride, neodymium gallate, and lithium gallate.
 7. The method of claim 6, wherein the seed layer is Si having a thickness less than about 3000 Å.
 8. The method of claim 1, wherein the support layer comprises a material from the group consisting of silicon carbide, aluminum nitride, silicon, and sapphire.
 9. The method of claim 1, wherein the seed layer and the support layer have substantially the same chemical composition.
 10. The method of claim 1, wherein the working layer is deposited onto the seed layer to a thickness of less than about 10 microns.
 11. The method of claim 1, wherein the method further includes applying an intermediate layer between the seed layer and the support.
 12. The method of claim 11, wherein the intermediate layer is a bonding layer or an insulating layer.
 13. The method of claim 1, wherein the seed layer includes a crystal lattice parameter sufficient for the epitaxial growth of the working layer onto the seed layer such that the working layer has a dislocation concentration less than about 10⁷/cm².
 14. The method of claim 1, further comprising providing a source substrate including the seed layer and a weakened zone; and detaching the seed layer from the source substrate at the weakened zone.
 15. The method of claim 14, wherein the weakened zone comprises implanted atomic species at a depth that corresponds to the thickness of the source substrate.
 16. The method of claim 14, wherein the seed layer is detached from the source substrate by application of heat treatment, mechanical stress, chemical etching, or a combination thereof.
 17. The method of claim 1, wherein the seed layer is prepared to receive the working layer, the preparation including polishing, annealing, smoothing, oxidation, and etching.
 18. The method of claim 1, further comprising removing the support substrate such that it remains in a condition sufficient for recycling and reusing the support substrate.
 19. A method for fabricating a substrate for optics, electronics, or opto-electronics, which method comprises: transferring a seed layer onto a support substrate; depositing a working layer on the seed layer to form a composite substrate; and detaching the seed layer and the working layer from the composite substrate; wherein the seed layer includes a crystal lattice parameter sufficient for epitaxial growth of the working layer having a dislocation parameter of less than about 10⁷/cm².
 20. The method of claim 19, wherein the seed layer is monocrystalline silicon carbide, the support substrate is polycrystalline silicon carbide, and the working layer is gallium nitride.
 21. The method of claim 19, further comprising a source substrate including the seed layer and a weakened zone; and further wherein the seed layer is detached from the source substrate at the weakened zone.
 22. The method of claim 21, wherein the weakened zone comprises implanted atomic species at a depth that corresponds to the thickness of the source substrate.
 23. The method of claim 19, further comprising removing the support substrate such that it remains in a condition sufficient for reuse.
 24. The method of claim 19, further comprising applying an intermediate layer between the seed layer and the support.
 25. The method of claim 24, wherein the intermediate layer is a bonding layer or an insulating layer. 